1. Field of the Invention
The present invention relates to a vehicle tire suitable for use on rough ground or terrain. More particularly, the invention relates to a vehicle tire having alternately arranged high-rigidity and low-rigidity areas with different degrees of radially-inward recession upon contact of the tire with the ground, so as to provide an increased degree of driving force on rough ground.
2. Description of the Art
Off-road vehicle tires adapted for driving on rough ground or terrain, such as gravel, sand, and other uneven surfaces, should have a specially designed tread so as to be comparable with tires to be used on paved roads with regard to driveability.
In FIG. 16 of the accompanying drawings there is shown a developed view of the tread pattern of a conventional tire 1 for use on rough ground. The cross-section of such tire 1 is shown in FIG. 17, as viewed vertically with the tire in an upright position. The tire 1 has a plurality of groups 2 of four projections 2a, 2b, 2c and 2d aligned with each other in the transverse or widthwise direction of the tire 1, and a plurality of groups 3 of five projections 3a, 3b, 3c, 3d and 3e aligned transversely with each other and spaced from the adjacent groups 2 in the circumferential direction of tire 1. The groups 2 and 3 are alternately arranged in the circumferential direction. With such arrangement, it is intended that as the tire 1 rotates the projections 2a through 2d, and 3a through 3e penetrate the gravel or the like of the rough ground, thereby producing driving forces for the tire.
On the other hand, it is desired, and has been proposed, that the tire be shaped so as to have a polygonal configuration with areas of larger and smaller radii upon contact with the ground. With such a structure, driving forces are transmitted when the sides of the polygonal tires contact the ground, and the tire is lifted when the corners of the polygonal tire act on the ground, whereby the vehicle can travel reliably on rough ground.
The conventional tire 1 above described, having the projection groups 2 and 3 alternately arranged circumferentially, has different degrees of rigidity because of the difference in the number of projections and the spacing between them. Although areas of a lower degree of rigidity become recessed radially inwardly to form a polygonal shape when the tire contacts the ground, the difference in rigidity between the projection groups is extremely small because the projections in each group are formed and disposed independently of each other. The projection groups 2 and 3 are provided at the tire tread, and the number of corners on the polygonal tire is extremely large, normally forty to fifty. Therefore, the tire rotates substantially in a circular shape, with the result that the aforesaid proposal is practically impossible to achieve. This result arises from the fact that the projections on the tire 1 of conventional design are provided for penetration into the ground with no concern for the above proposal for reliable travel on rough ground.
As shown in FIG. 18, because the projections 3b, 3d are located adjacent to and on each side of central projection 3c, the distances L.sub.1, L.sub.1, respectively, between the projections 3b, 3c and between the projections 3c, 3d change only slightly even if the tire contacts the ground. When the tire 1 rotates on rough ground, the gravel or other upper layer of ground does not collect toward the central projection 3c, but is displaced alongside the tire. When, on the other hand, the groups 2 contact the ground as illustrated in FIG. 19, the weight of the vehicle is borne by the centrally located projections 2b, 2c. Although the distance L.sub.2 between the projections 2b, 2c is larger than the distance L.sub.1, the distance L.sub.2 does not change greatly either, with the result that the gravel or the like is shifted alongside the tire rather than centrally gathered.
The second projection 3b counting from the outermost projection 3a in the group 3 has an outer surface 3b' which does not extend vertically (FIG. 20) and has its upper edge 3b''' recessed inwardly widthwise of the tire to a greater extent than its lower edge 3b". Thus, between the outermost projection 3a and the next projection 3b, there is formed a space 4 into which no gravel or sand enters while the tire contacts the ground as the vehicle runs thereon. Accordingly, the distance L.sub.3 over which the gravel or sand is held in contact with the inner surface 3a' of the outermost projection 3a substantially corresponds to the lateral distance between the edge 3a" of the projection 3a and the edge 3b" of the projection 3b. The surface 3a' does not contact the gravel or sand over its entire width, and hence cannot function to stiffen the ground material, leaving insufficient reactive forces available for driving the tire. Therefore, there is a limitation on driving forces produced between the conventional tire 1 and the ground, and at times the tire 1 tends to slip under bad ground conditions. On cornering, when the vehicle changes its running direction, the edge 3a" of the outermost projection 3a only functions to prevent sidewise slipping, and driving and accelerating forces are provided by the surfaces 5 of the projections 3a through 3c, with the inner surface 3a' of the projection 3a failing to effectively serve such purpose during cornering. Thus, the surface 3a' of the projection 3a fails to gain an increased degree of driving force.
The present invention provides a pneumatic vehicle tire for use on rough ground or terrain, such as in sand and/or gravel, which eliminates the foregoing problems associated with conventional vehicle tires.